1. Field of the Invention
The present invention relates to a discharge lamp having a discharge path-defining member which defines the discharge path in the bulb as a convoluted path with an interlinked section running between the top and the bottom of the bulb.
2. Description of the Prior Art
In recent years, a number of small fluorescent lamps have been proposed which feature the merits (high efficiency, long life) of discharge lamps such as fluorescent lamps, and which are compact in size and which can therefore be enployed in locations in which typically incandescent lamps have been used.
FIG. 1 shows an example in which a pair of electrodes 15a, 15b are provided in an opposed manner on one side of a cylindrical bulb 1 made of a glass, and the discharge path established between the electrodes 15a and 15b is caused to run in a convoluted manner with an interlinked passage running between the top and bottom of the bulb 1 by a discharge path-defining member 5 which is formed substantially in the shape of a cross.
Unless the discharge path-defining member 5 makes positive contact with the inner surface of the bulb 1, however, discharging may take place between adjacent sections of the discharge path, making it difficult to obtain a discharge path which follows the desired convoluted path. It is particularly difficult for the discharge path-defining member 5 to make positive contact with the bulb 1 at the top portion 3 of the bulb 1. Therefore, bulbs in which the top 3 is flat such that it comes into contact with the upper end of the discharge path-defining member 5 as shown in FIG. 1, have been tried, and, as shown in FIG. 2, forming one of the plates of the discharge path-defining member such that its top 9 makes positive contact with the inner surface of the top 3 of the bulb, which is formed in the shape of a dome, has also been tried. With the former bulb of FIG. 1, however, the circumferential edge of the flat plane of bulb 1 tends to be thick which makes it difficult for the discharge path-defining member to make intimate contact therewith. Further, residual stresses in the thickened portions are a frequent cause of breakage during and after production.
Moreover, the end 9 of the discharge path-defining member 5 forms a shadow on the flat plane, and the flat top 3 gives the bulb a poor external appearance.
With the latter bulb shown in FIG. 2, on the other hand, the top 3 of the bulb is formed in a dome shape, and less of a shadow is formed by the top 9 of the discharge path-defining member 5. Further the appearance is better than that of the former bulb. However, it is very difficult to make the top 9 of the discharge pathdefining member 5 positively contact the inner surface at the top 3 of the bulb because of imperfection in the shape of dome. Therefore, it is difficult to industrially produce the bulbs.
In view of the above-mentioned circumstances, therefore, a bulb has been proposed as shown in FIG. 3, which maintains the shape of a dome, which eliminates the need for bringing the discharge path-defining member 5 into contact with the curved dome portion, and which has an industrially desirable appearance. That is, a circular top plate 12 that comes into contact with the inner circumference of the bulb 1 is attached to the top 9 of the discharge path-defining member 5 which has the same shape as that of FIG. 1. Even if the top plate 12 is made of a light-transmitting material, however, ultraviolet rays produced by the discharge are reduced by the top plate 12, and only a small amount of light is emitted in the dome-shaped portion, since the discharge passes beneath the top plate 12. This means that when the lamp is mounted with its base 18 upwards, the area directly under the lamp is poorly illuminated. In other words, there are limitations on the use of the lamp.